Exhaust valve steel



Feb. 6, 1962 c, w, VIGORI 3,020,152

EXHAUST VALVE STEEL Filed May 2, 1960 O.63O.68% CARBON 8.75-IO.25%MANGANESE 20.5-22.5% CHROMIUM 2.2 2.8% MOLYBDENUM O.34-O.38% NITROGEN0.02-0.03% BORON O--O.5% SILICON BALANCE IRON IN VEN TOR.

ATTORNEY 3,020,152 EXHAUST VALVE STEEL Charles W. Vigor, East Detroit,Mich., assignor to General Motors Corporation, Detroit, Mich., acorporation of Delaware Filed May 2, 1960, Ser. No. 26,293 7 Claims.(Cl. 75-126) This invention relates to an austenitic stainless steelhaving outstanding high-temperature properties and particularly to aninternal combustion engine exhaust valve formed of such a steel.

During the past few years numerous stainless steels, includingaustenitic stainless steels, have been developed and used commerciallyfor high-temperature applications. Exhaust valves for internalcombustion engines, for example, have been formed of austeniticstainless steels containing high alloy contents. However, none of thesesteels has been completely satisfactory in all respects as an exhaustvalve material. While some of the alloy steels commercially availabletoday have a desirable low nickel content and low alloy contentgenerally, these steels do not possess the hardness necessary to avoidexcessive tip wear in single-piece valves. Other valve steels haveinadequate high-temperature strength or poor workability.

Accordingly, it is a principal object of the present invention toprovide an austenitic stainless steel which completely satisfies todaysstringent physical property requirements for exhaust valves of gasolineengines but which has a relatively low strategic alloy content. Theaustenitic stainless steel hereinafter described possesses the desirablecombination of excellent high-temperature strength, oxidation resistanceand room-temperature hardness while retaining a satisfactory degree offorgeability.

Other objects and advantages of this invention will more fully appearfrom the following detailed description of a preferred embodiment,reference being made to the accompanying drawing which shows a poppetvalve for an automobile engine.

The exceptionally high stress-rupture strength of the austeniticstainless steel described herein results from a balanced composition inwhich the percentage ranges of the various elements present are specificand relatively narrow. This stainless steel consists essentially ofabout 0.63% to. 0.68% total carbon, silicon not in excess of 0.5%, 8.75%to 10.25% manganese, 20.5% to 22.5% chromium, 2.2% to 2.8% molybdenum,0.34% to 0.38% nitrogen, 0.02% to 0.03% boron and the balance iron.Outstanding results have been obtained with alloy steel exhaust valveshaving a nominal chemical composition of approximately 0.65% carbon,0.25% silicon, 9.5% manganese, 21.5% chromium, 2.5% molybdenum, 0.36%nitrogen, 0.025% boron and the balance iron. It will be noted that,unlike most of the austenitic stainless steels heretofore usedcommercially in exhaust valves of automobile engines, the alloydescribed herein need not contain nickel, a relatively expensivestrategic metal. The manganese and nitrogen in the aforementionedamounts promote austenite stability even in the absence of nickel.

The specific combination of constituents in the limited ranges set forthabove provides the austenitic stainless steel alloy, after relativelysimple heat treatment, with exceptional room-temperature hardness aswell as excellent high-temperature strength. A Rockwell C-hardnitedStates Patent "ice ness of at least 38 results from solution treatmentplus aging. The various elements in the alloy steel are balanced so asto maintain a high level of strength during long periods of exposure atelevated temperatures.

Carbon is present in the specific range set forth above to perform twofunctions. Carbon is necessary to provide a stable austenitic phase, andit also contributes to the strength and hardness of the steel at bothroom and elevated temperatures by forming carbides with otherconstituents in the alloy. As indicated above, the best combination ofthese effects is obtained if the aforementioned carbon range is rigidlyadhered to. If the carbon content-is reduced below approximately 0.63%,the roomtemperature hardness of the steel is decreased. An excessivelyhigh carbon content reduces its high-temperature stress-rupturestrength.

When the amount of silicon present does not exceed about 0.5%, it doesnot appreciably afiect the strength or stability of the alloy. Ingeneral, a higher silicon content improves the oxidation resistance ofstainless steel alloys. However, in the case of exhaust valve steels, ithas been found that a high silicon content actually is detrimental tooxidation resistance since it is incompatible with lead oxide enginedeposits.

Manganese is added to the stainless steel principally for its beneficialeffect in promoting a stable austenitic structure. It has been foundadvantageous to include only that amount of manganese which, incombination with the other austenite stabilizing elements (carbon andnitrogen), results in maximum long-term stability. If the quantity ofmanganese present in the alloy is less than approximately 8.75%, thehigh-temperature strength and oxidation resistance are reduced.

The oxidation resistance of the stainless steel is further increased toan appreciable extent by the presence of chromium. In the case ofexhaust valves, the chromium content should be at least 20.5% to providethe desired oxidation resistance. The maximum amount of chromium whichmay be present is determined by the advisability of maintaining theproper austenite-fern'te balance since chromium has a pronounced eifectin promoting ferrite formation. The chromium level is one of the majorfactors in determining th amount of austenite stabilizing elements(carbon, manganese and nitrogen) required to stabilize an austeniticstructure. If the chromium content of the steel is increased withoutproperly adjusting the amounts of the other elements, it produces astructure which is unstable during long periods of exposure at elevatedtemperatures. The resultant instability, which is manifested by atransformation into a gradually increasing amount of ferrite, reducesthe hightemperature strength of the alloy steel.

Chromium also enters into reactions with carbon and iron to form complexcarbides which increase the strength and hardness of the stainless steelat both room and elevated temperatures. It should be noted that chromiumrestricts the formation of austenite under hightemperature equilibriumconditions. However, it anomalously also promotes the retention ofaustenite at low temperatures because of the sluggishness it imparts tothe kenetics of the austenite-ferrite phase transformation.

The molybdenum contributes principally to the hightemperature strengthof the steel through solution hardening of the austenitic matrix. Sincemolybdenum also tends to promote the formation of ferrite, the carbon,manganese and nitrogen contents should be adjusted accordingly.

The stainless steel alloy described herein contains nitrogen the amountspecified because of the austenite-stabilizing influence of thiselement. If nitrogen is omitted, the cmbon and manganese in the steelare ineifective to maintain the alloy as a stable austenitic material.As in the case of manganese, it has been found desirable to keep thenitrogen content at the lowest level commensurate with the requirementfor phase stability. A nitrogen content in excess of approximately 0.38%lowers the hightemperature strength of the steel and results inundesirable carbide precipitation reactions. These reactions produce alamellar type of carbide precipitate rather than a uni form dispersionof carbide. It appears that the nitrogen is not in the precipitate butremains in the parent austenite phase.

Boron is included in the present stainless steel alloy to improve thelong-time high-temperature strength and ductility. It is believed thatboron preferentially locates in the grain boundary areas of the alloyand prevents the precipitation of carbides in these areas. Consequently,it is possible for the resultant stainless steel to sustain creep typeof deformation for a longer period of time without initiating rupturesat grain boundaries. Such ruptures normally indicate the beginning ofhigh-temperature stress-rupture type of failure. A boron content lessthan about 0.02% does not provide the desired high-temperaturestrengthand ductility, while the addition of boron in an amount greater thanapproximately 0.03% is detrimental to forgeability and oxidationresistance. The resultant decrease in forgeability causes hot shortness,while the reduction in oxidation resistance is evidenced by anonadherent oxide which forms at high temperatures.

The initial level'of strength in this alloy is dependent upon controlledprecipitation of complex carbides from a solid solution matrix. This isaccomplished by heat treatment, the strength being determined by thetype and amount of the precipitate in relation to the nature of thematrix. Maintaininga high level of strength at elevated temperaturesover a long period of time requires that precipitation reactions andmatrix stnlcture composition remain in an optimum balance. Since it isobviously im- -possible to readjust the chemical composition of anexhaust valve while in service, the optimum balance for long-termproperties must be provided by the original chemical composition, ofcourse. As indicated above,

this goal has been achieved in thepresent austenitic steel alloy toanoutstanding degree.

The-principal problem in achieving the aforementioned balance involvesmaintaining the proper amounts of those elements which promote theformation and stabilization of an austenitic structure in relation tothose which favor the formation of ferrite. In the above-describedstainless steel the carbon, manganese and nitrogen tend to produce astable austenitic structure, while the chromium and molybdenum favor theformation of ferrite. This delicate balance is further complicated bythe requirement for precipitation hardening because some of these sameelements affecting the austenite-ferrite balance also react to form theprecipitating compounds. These compounds consist principally of complexcarbides of iron and chromium. Since precipitation reactions are notlimited to the initial heat treatment of a valve, but continue duringits entire service life due to the high operating temperatures in thecombustion chamber of a gasoline engine, the proper determination of acritical composition balance must be based on a timedependent,high-temperaing. At 1400 F, the stress for the same rupture life isstill normally in the range between 26,000 p.s.i. and 27,500 p.s.i. Evenwhen the stainless steel is heated to a temperature of 1500" F, thestress for a l00-hour rupture life is about 15,500 p.s.i. to 17,000p.s.i. This maximum stress for hours remains in the 9,200 p.s.i. to10,300 p.s.i. range when the temperature is increased to 1600 F. Itshould be noted that these excellent stress-rupture properties areobtained without sacrificing the forgeability and hardness required forexhaust valves of gasoline engines. The method of preparing the alloysteel is entirely conventional.

This austenitic stainless steel can be used in a wide variety ofcommercial applications because of its good workability. It can beforged or extruded Without 'difiiculty. Furthermore, the aforementionedhigh room-temperature hardness of the steel permits singlepiece valves,Which are very resistant to tip Wear, to be formed of it. As indicatedabove, this steel also exhibits high-temperature strength considerablyabove that obtained with other valve steel alloyshaving comparable andeven greater total alloy contents.

. While my invention has been described by means of certain-specificexamples, it is to be understood that its scope is not to be limitedthereby except as defined by the following claims.

.I claim:

1. A poppet valve for an internal combustion engine, said valve beingformed of a substantially nickel-free,

.austenitic stainless steel consisting essentially of about essentially.of about 0.63% to 0.68% carbon, 0% to 5% silicon, 8.75% to 10.25%manganese, 20.5% to 22.5% chromium, 2.2% to 2.8% molybdenum, 0.34% to0.38%

nitrogen, 0.02% to 0.03% boron and the balance substantially all iron,said valve having a stress rupture strength ofat least 26,000 p.s.i. for100 hours at a temperature of 1400 F.

3. A one'piece, solution-treated and aged exhaust valve for a gasolineengine, said valve having a Rockwell C hardness at room temperature ofat least 38 and being formed of an austenitic stainless steel consistingessentially of about 0.65% carbon, 0.25% silicon, 9.5% manganese, 21.5%chromium, 2.5% molybdenum, 0.36% nitrogen, 0.02% boron, and the balancesubstantially all iron, said valve having a stress-rupture strength ofat least 26,000 p.s.i. for 100 hours at a temperature of 1400 F.

4. An austenitic stainless steel characterized by outstandinghigh-temperature strength, oxidation resistance and room-temperaturehardness, said stainless steel consisting essentially of about 0.63 to0.68% carbon, 8.75%

to 10.25% manganese, 20.5 to 22.5 chromium, 2.2% to 2.8% molybdenum,0.34% to 0.38% nitrogen, 0.02% to 0.03% boron'and the balancesubstantially all iron.

5. A forgeable, nickel-free, austenitic stainless steel characterized byoutstanding high-temperature strength, oxidation resistance androom-temperature hardness, said stainless steel consisting essentiallyof about 0.63% to 0.68% carbon, silicon not in excess of 0.5%, 8.75% to10.25% manganese, 20.5 to 22.5% chromium, 2.2% to 2.8% molybdenum, 0.34%to 0.38% nitrogen, 0.02% to 0.03% boron and the balance substantiallyall iron.

6. A heat-treated, forgeable, austenitic stainless steel characterizedby outstanding oxidation resistance, room temperature hardness and astress-rupture life of at least 100 hours under a stress of 26,000p.s.i. at a temperature of 1400 F., said stainless steel consistingessentially of about 0.63% to 0.68% carbon, 0.1% to 0.5% silicon, 8.75%to 10.25% manganese, 20.5 to 22.5 chromium, 2.2% to 2.8% molybdenum,0.34% t0:0.38% nitrogen,

JIM

having a stress-rupture life of at least 100 hours under a stress of26,000 p.s.i. at a temperature of 1400 F.

References Cited in the file of this patent UNITED STATES PATENTSDyrkacz et a1 Sept. 25, 1956 Dyrkacz et al Nov. 26, 1957 MacFarlans eta1. Aug. 9, 1960

7. A SOLUTION-TREATEDF AND AGED AUSTENITIC STAINLESS STEEL HAVING AROCKWELL"C" HARDNESS AT ROOM TEMPERATURE OF AT LEAST 38 AND CONSISTINGESSENTIALLY OF ABOUT 0.63% TO 0.68% CARBON, 0% TO 5% SILICON, 8.75% TO10.25% MANGANESE, 20.5% TO 22.5% CHROMIUM, 2.2% TO 2.8% MOLYBDENUM,0.34% TO 0.38% NITROGEN,0.02% TO 0.03% BORON AND THE BALANESUBSTANTIALLY ALL IRON, SAID STEEL HAVING A STRESS-RUPTURE LIFE OF ATLEAST 100 HOURS UNDER A STRESS OF 26,000 P.S.I. AT A TEMPERATURE OF1400*F.